Measure the flour, salt and baking powder into a bowl, mix and set aside.

In a large bowl, cream together the softened butter and the sugar. Once smooth, add the eggs, vanilla and combine well.

Add the flour mixture to the butter / egg mixture a little at a time. Mix until the dough is smooth and flour is fully incorporated.

Cover the dough with plastic wrap so that it is touching the surface of the dough. Chill in the refrigerator for 1 hour. Overnight is fine too. If you want to freeze the dough, you can do so at this stage.

Divide and place in two separate bowls.

Knead food colouring into one of the parts of dough. I made one half deep blue and left the other half as is. Chill the dough.

On a lightly floured surface, roll out each ball of dough separately to about 1/4 inch thickness.

Using a pastry brush, apply a little water to the darker layer of dough. Roll the lighter colour of dough around the rolling pin and transfer it on top of the darker coloured layer of dough. The little bit of water will help them stick together.

Trim the edges so you end up with a neat and tidy rectangle. Save the scraps! You can still bake the scraps up into yummy cookies! They'll just look 'marbled'.

Gently start rolling your dough from one of the wider ends, brushing a we bit of water on the dough as you roll it up. Once it's rolled up, you can work the 'log' to even it out if the middle is thicker then the ends, and to work out any gaps.

Spill out some sprinkles onto the counter, brush the outside of the dough 'log' with a bit more water and roll it in the sprinkles. I used an entire container of sprinkles to coat the outside. Finally, wrap the dough in plastic wrap and refrigerate for an hour. You could also freeze the dough at this point to bake at a later date

Once chilled, slice the dough into rounds to your desired thickness. I like them a little over 1/4 inch thick. The thinner the blade used to slice them, the better. A thick blade will tend to squish the dough.

Bake in the centre of a preheated oven at 350 F for approximately 8 minutes (depending on the size and thickness). Here is the trick: As soon as the cookies no longer look 'wet' on the surface, they are done! Voila! Pretty, swirly cookies!